The invention relates to a method of ion implantation with an ion source material for generating boron ions.
As in semiconductor technology the demand for microminiaturization and ever closer tolerances at very small dimensions is increasing, so is the method of ion implantation, in particular for doping semiconductor materials, gaining in importance. With the ion implantation method, ions of an accurately defined energy are injected into the semiconductor material, whereby it is possible to predetermine equally accurately the depth of penetration of the injected ion and the lateral expansion of the area to be doped. In addition, this method, in comparison with the previously employed diffusion methods, permits the doping profile to be influenced almost arbitrarily.
With the ion sources generally employed the ions used for implantation are generated by a gas discharge into which the ion source material is injected in a gaseous form. For p conductivity generation, semiconductor technology generally employs boron doping. Known ion source materials for generating boron ions are boron halides, in particular boron fluroide, boron chloride, and boron bromide. These compounds which are gaseous and highly volatile permit the generation of ion currents, as are required for an economical, i.e., an ion implantation under manufacturing conditions.
The Sidenius type ion source, which is described in detail further on and which is distributed by the Danfysik company under the name "High-Temperature, Hollow-Cathode, Ion Source", modified Type 911, for example, in conjunction with boron halides as an ion source material, permits generating ion currents of the order of 40 microamps. On the other hand, however, boron halides are extremely poisonous. During their decomposition, as, for example, in gas discharges, the highly corrosive halogens are freed, resulting in the ion source and other apparatus parts, such as pumping equipment, being destroyed. For these reasons the use of boron halides as ion source material is subject to very strict safety regulations. Therefore, attempts have been made to find boron ion source materials which are non-poisonous and which generate only a minimum quantity of corrosive decomposition products. Elementary boron is a boron ion source material fitting this description. With elementary boron, the boron is heated in an oven connected to the ion source. However, even at the maximum temperatures, about 1500.degree. C, to which conventional ovens may be subjected, the ion currents yielded by the above-mentioned ion source, using boron as boron ion source material, are only of the order of 1 microamp. This would mean that an ion implantation process taking, for example, 1 minute, using boron halides as boron ion source material would last 30 to 40 minutes with boron as boron ion source material. As implantation times of such length are both unacceptable and uneconomical, elementary boron, in spite of its otherwise favorable characteristics as boron ion source material, is impracticable for manufactural applications.